Generally, in a transmittance densitometer, the measuring area or the diameter of an aperture path is 0.5 mm (about 0.0197 inches), 1.0 mm (about 0.039 inches) or 2.0 mm (about 0.078 inches). When designing densitometers of this type, the size of the light receiving diffuser disposed on the measuring light path is selected to be larger than the diameter of a maximum aperture in consideration of reliable transmission of light from the aperture to the diffuser.
In order to be able to visually confirm what portion of a sample is measured, a sample stage is made from a semi-transparent light-diffusion plate and illuminated interiorly. With this sample stage illuminated interiorly, however, the light illuminating the sample stage impinges, upon the diffuser to give rise to degradation in measurement accuracy because the outer diameter of the diffuser is far larger than the diameter of the aperture. Therefore, in a conventional light transmission type densitometer, the sample stage is provided on its surface with an apertured opaque disc of about 10 mm (about 0.39 inches) diameter surrounding the measuring light path.
The apertured opaque disc, however, prevents the neighbourhood of the measuring location on a film from being illuminated, thereby giving rise to difficulties in visual confirmation of the measuring location. These difficulties are fatal when the optical density at the measuring location is approximately to that of its neighbourhood. Thus, an operator tends to perform measurement of an unintended portion of a sample.